This invention is directed to coating metal elements with a thin film of calcium nitrite (including compositions containing same) whereby the metal is inhibited against corrosion on being placed in hydraulic (including alite) cement.
The binder component in the cements, mortars and concretes used widely as a construction material is portland cement. Portland cement is manufactured by calcining a mixture of limestone and clay to form a clinker, and by grinding the clinker to a fine powder. The major compounds found in portland cement are tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. The tricalcium and dicalcium silicates are thought to be the principal bonding constituents in the portland cement. Tricalcium silicate, when mixed with water, forms a calcium silicate hydrate known as tobermorite gel and calcium hydroxide. The dicalcium silicate, when contacted with water, forms similar products but at a much lower rate of reaction. The tricalcium silicate, having the greater rate of reaction determines, to a large extent, the hardening rate of the cement. To provide materials which are suitable for different uses, portland cements having a range of hardening rates have been found desirable. By producing cements having a range of proportions of tricalcium silicate present, a range of hardening rates and strength development has been obtained. Four general types of portland cements, varying principally in the relative quantities of tricalcium silicate and dicalcium silicate present therein, are commonly produced. The proportions of the principal compounds present in each type of cement are shown in Table I.
TABLE I ______________________________________ Cement type I II III IV ______________________________________ Composition, wt. percent: Tricalcium silicate 53 47 58 26 Dicalcium silicate 24 32 16 54 Tricalcium aluminate 8 3 8 2 Tetracalcium aluminoferrite 8 12 8 12 ______________________________________
The term "alite cement" is defined as including neat pastes, mortars, and concretes and the mixed, dry unreacted ingredients of neat pastes, mortars, and concretes, comprising as alite cement binder, a composition containing greater than 20 percent tricalcium silicate based on the dry weight of the composition. The most common alite cements are portland cements, and mortars and concretes containing portland cements. Most commerically available alite cements contain binders comprising from about 20 to 75 percent tricalcium silicate. The alite cement binder, or concrete binder, is the component which provides the desired bonding, for example, portland cement.
Hydraulic cements (including alite cements) encounter various corrosion environments. In some, the environment is an inherent part of the cement, e.g., as by use of calcium chloride accelerator, or the use of chloride containing materials. Other environments may be extraneous, e.g., use of calcium chloride and/or salt in snow and ice removal, exposure to salt spray or brines, and the like. Such environments tend to attack and corrode metal pieces within or in contact with the alite cement. The instant invention provides coatings for the metal that we believe inhibit such corrosion.